This invention relates to multidirectional drainage mats which are useful and effective, for instance as a highway edge drain for the dewatering of highway pavement systems.
The problem of water in pavements has been of concern to engineers for a considerable period of time. As early as 1823 McAdam reported to the London (England) Board of Agriculture on the importance of keeping the pavement subgrade dry in order to carry heavy loads without distress. He discussed the importance of maintaining an impermeable surface over the subgrade in order to keep water out of the subgrade.
The types of pavement distresses caused by water are quite numerous. Smith et.al. in the "Highway Pavement Distress Identification Manual" (1979) prepared for the Federal Highway Administration of the U.S. Department of Transportation identifies most of the common types of distresses.
Moisture in pavement systems can come from several sources. Moisture may permeate the sides, particularly where coarse-grained layers are present or where surface drainage facilities within the vicinity are inadequate. The water table may rise; this can be expected in the winter and spring seasons. Surface water may enter joints and cracks in the pavement, penetrate at the edges of the surfacing, or percolate through the surfacing and shoulders. Water may move vertically in capillaries or interconnected water films. Moisture may move in vapor form, depending upon adequate temperature gradients and air void space. Moreover, the problem of water in pavement systems often becomes more severe in areas where frost action or freeze-thaw cycles occur, as well as in areas of swelling soils and shales.
The types of pavement distresses caused by water are quite numerous and vary depending on the type of pavement system. For flexible pavement systems some of the distresses related to water either alone or in combination with temperature include: potholes, loss of aggregates, raveling, weathering, alligator cracking, reflective cracking, shrinkage cracking, shoving, and heaves (from frost or swelling soils). For rigid pavement systems, some of the distresses include faulting, joint failure, pumping, corner cracking, diagonal cracking, transverse cracking, longitudinal cracking, shrinkage cracking, blowup or buckling, curling, D-cracking, surface spalling, steel corrosion and heaving (from frost or swelling soils).
Similar types of distresses occur in taxiways and runways of airfields.
Numerous of these joint and slab distresses are related to water pumping and erosion of pavement base materials used in rigid pavement construction. Water pumping and erosion of pavement base materials have been observed to cause detrimental effects on shoulder performance as well. Also, many of the distresses observed in asphalt concrete pavements are caused or accelerated by water.
For instance, faulting at the transverse joints is a normal manifestation of distress of unreinforced concrete pavements without load transfer. Faulting can occur under the following conditions:
1. The pavement slab must have a slight curl with the individual slab ends raised slightly off the underlying stabilized layer (thermal gradients and differential drying within the slab create this condition).
2. Free water must be present.
3. Heavy loads must cross the transverse joints first depressing the approach side of the joint, then allowing a sudden rebound, while instantaneously impacting the leave side of the joint causing a violent pumping action of free water.
4. Pumpable fines must be present (untreated base material, the surface of the stabilized base or subgrade, and foreign material entering the joints can be classified as pumpable fines).
Faulting of 1/4 in. or more adversely affected the riding quality of the pavement system.
Methods for predicting and controlling water contents in pavement systems are well documented by Dempsey in "Climatic Effects on Airport Pavement Systems--State of the Art", Report No. FAA-RD-75-196 (1976), U.S. Department of Defense and U.S. Department of Transportation. Methods for controlling moisture in pavement systems can generally be classified in terms of protection through the use of waterproofing membranes and anticapillary courses, the utilization of materials which are insensitive to moisture changes, and water evacuation by means of subdrainage.
Field investigations indicate that evacuation by means of a subdrainage system is often the preferred method for controlling water in pavement systems. In this regard proper selection, design, and construction of the subdrainage system is important to the long-term performance of a pavement. A highway subsurface drainage system should, among other functions, intercept or cut off the seepage above an impervious boundary, draw down or lower the water table, and/or collect the flow from other drainage systems.
Existing highway drains include a multitude of designs. Among the simplest are those which comprise a perforated pipe installed at the bottom of an excavated trench backfilled with sand or coarse aggregate. For instance, a standard drain specified by the State of Illinois requires a 4-inch diameter perforated pipe be placed in the bottom of a trench 8 inches (20.3 cm) wide by 30 inches (76 cm) deep. The trench is then backfilled with coarse sand meeting the State of Illinois standard FA1 or FA2. Such drains are costly to fabricate in terms of labor and materials. For instance the material excavated from the trench must be hauled to a disposal site, and sand backfill must be purchased and hauled to the drain construction site.
Other types of drains have attempted to avoid the use of the perforated pipe by utilizing a synthetic textile fabric as a trench liner. The fabric lined trench is filled with a coarse aggregate which provides a support for the fabric. The void space within the combined aggregate serves as a conduit for collected water which permeates the fabric. Such drains are costly to install, for instance in terms of labor to lay in and fold the fabric as well as in terms of haulage of excavated and backfill material. Moreover, there is considerable fabric area blocked by contact with the aggregate surface. This results in an increased hydraulic resistance through the fabric areas contacting the aggregate surface.
Other modifications to drainage material include fabric covered perforated conduit, such as corrugated pipe as disclosed by Sixt et.al. in U.S. Pat. No. 3,830,373 or raised surface pipe as disclosed by Uehara et.al. in U.S. Pat. No. 4,182,581. A disadvantage is that the planar surface area available for intercepting subsurface water is limited to approximately the pipe diameter unless the fabric covered perforated conduit is installed at the bottom of an interceptor trench filled, say, with coarse sand. A further disadvantage is that much of the fabric surface, say about 50 percent, is in contact with the conduit, thereby reducing the effective collection area.
The problem of limited planar surface area for intercepting subsurface water is addressed by drainage products disclosed by Healy et.al. in U.S. Pat. Nos. 3,563,038 and 3,654,765. Healy et.al. generally disclose a planar extended surface core covered with a filter fabric which serves as a water collector. One edge of the core terminates in an pipe-like conduit for transporting collected water. Among the configurations for the planar extended core are a square-corrugated sheet and an expanded metal sheet. A major disadvantage of designs proposed by Healy et.al. is that the drains are rigid and not bendable; this requires excavation of sufficiently long trenches that an entire length of drain can be installed. The pipe-like conduit requires a wider trench than might otherwise be needed. Moreover, the expanded metal sheet core does not provide adequate support to the fabric which can readily collapse against the opposing fabric surface, thereby greatly reducing the flow capacity within the core. Also the square corrugated sheet core is limited in that at least 50 percent of the fabric surface arc is occluded by the core, thereby reducing water collection area.
A related drainage material with extended surface is a two-layer composite of polyester non-woven filter fabric heat bonded to an expanded nylon non-woven matting such as ENKADRAIN.TM. foundation drainage material available from American Enka Company of Enka, N.C. The drainage material which can be rolled has filter fabric on one side of the nylon non-woven matting. The drainage material serves as a collector only and requires installation of a conduit at the lower edge. This necessitates costly excavation of wide trenches, in addition to cost of conduit.
Another related drainage material with extended surface comprises a filter fabric covered core of cuspated polymeric sheet, such as STRIPDRAIN drainage product available from Nylex Corporation Limited of Victoria, Australia. The impervious cuspated polymeric sheet divides the core into two isolated opposing sections which keeps water collected on one side on that side. Moreover, in order that the drainage material be flexible, the core must be contained in a loose fabric envelope, which being unsupported on the core can collapse due to soil loading into the core thereby blocking flow channels. The cuspated polymeric sheet is bendable only along two perpendicular axes in the plane of the sheet. This makes installation somewhat difficult, for instance whole lengths must be inserted at once in an excavated trench.
A still further similar polymeric drainage product comprises a perforated sheet attached to flat surfaces of truncated cones extending from an impervious sheet, such as CULDRAIN board-shaped draining material available from Mitsui Petrochemical Industries, Ltd. The perforated sheet has holes in the range of 0.5 to 2.0 millimeters in diameter and allows fine and small particles to be leached from the subsurface soil.
The drainage materials available have one or more significant disadvantages, including economic disadvantages of requiring extensive amounts of labor for installation and performance disadvantages such as requiring separate conduit for removing collected water. A further performance disadvantage is that the drainage materials utilize fabric which, depending on the adjoining soil, may become blinded with soil particles or may allow too much material to pass through resulting in loss of subgrade support.
This invention overcomes most if not all of the major disadvantages of engineering fabric utilized in previously known drainage materials.
Among the useful parameters for characterizing fabric useful in the drainage mat of this invention is the coefficient of permeability which indicates the rate of water flow through a fabric material under a differential pressure between the two fabric surfaces expressed in terms of velocity, e.g., centimeters per second. Such coefficients of permeabiity can be determined in accordance with American Society for Testing and Materials (ASTM) Standard D-737. Because of difficulties in determining the thickness of a fabric for use in determining a coefficient of permeability, it is often more convenient and meaningful to characterize fabric in terms of permittivity which is a ratio of the coefficient of permeability to fabric thickness, expressed in terms of velocity per thickness, which reduces to inverse time, e.g., seconds.sup.-1. Permittivity can be determined in accordance with a procedure defined in Appendix A of Transportation Research Report 80-2, available from the U.S. Department of Transportation, Federal Highway Administration.
Engineering fabrics used with drainage mats can be quite effective in protecting soil from erosion while permitting water to pass through the fabric to the conduit part of the drainage mat. However, the fabric must not clog or in any way significantly decrease the rate of flow. At the same time the fabric must not let too much material pass through, or clogging of the drainage mat could occur. However, loss of subgrade support could also occur.
When considering the actual soil-filter fabric interaction, a rather complex bridging or arching occurs in the soil next to the fabric that permits particles much smaller than the openings in the fabric to be retained. Failure of the soil-fabric system can result from either excessive piping of soil particles through the fabric or from substantial decrease in permeability through the fabric and adjacent soil.
The use of engineering fabrics in highway drainage mats requires the consideration of an additional factor. A highway is subjected to repeated dynamic loading by traffic. Such loading can lead to substantial pore pressure pulses in a saturated pavement system. During and after heavy rain a soil-filter fabric at the pavement edge may be subjected not only to a static hydraulic gradient, but also to a dymanic gradient caused by the highway traffic loading.
In this regard another useful parameter for characterizing fabric useful in the drainage mat of this invention is "dynamic permeability" which indicates the rate of water flow through a column of specifically gradated soil over a layer of fabric material under a combined static and dynamic hydraulic gradient. "Dynamic permeability" characterizes fabric performance in resisting blinding and pluggage under conditions which duplicate the effects of repeated traffic loading. The method for determining "dynamic permeability" is disclosed in Example II, herein.